Adsorption and separation of CH4/N2 by electrically neutral skeleton AlPO molecular sieves

Abstract

Four electrically neutral framework porous (AlPO) molecular sieves, UiO-7, AlPO4-33, AlPO4-17, and AlPO4-18, as all-silicon molecular sieve isoelectronic analogues, have been synthesized and studied with regard to their performance in CH4/N2 adsorption and separation. Experimental results have shown that UiO-7, with the topological type ZON, has the lowest Brunauer–Emmett–Teller (BET) surface area, but exhibits the highest CH4 adsorption capacity of 0.82 mmol/g and the highest adsorption selectivities of 4.4 (50:50, v/v) and 4.2 (20:80, v/v) among the four materials. BET surface area has been evaluated from N2 adsorption and desorption isotherms at 77 K, and the average pore sizes of the four materials have been calculated on the basis of a density functional theory (DFT) model. Interestingly, the CH4 adsorption capacity and adsorption selectivity have been found to first increase and then decrease with increasing average pore size of the four AlPO materials (AlPO4-18: 0.56 nm, AlPO4-17: 0.69 nm, UiO-7: 0.77 nm, AlPO4-33: 0.88 nm). Compared with the data of activated carbon material that also has no surface adsorption sites, it is found: They were maximized when the average pore size of the adsorbent was close to 0.76 nm (diameter of CH4, 0.38 nm). Theoretical calculations revealed that the suitable pore size of UiO-7, match better with CH4 to provide stronger binding energy with CH4 than other selected materials. And this result is verified by GCMC simulation. Thus, we infer that this is why UiO-7 shows the highest CH4 selectivity. Mixed-gas breakthrough experiments and PSA simulations have further proved that UiO-7 may be used for CH4/N2 adsorption separation, and indeed is the best choice among the studied AlPO molecular sieves.